Abstract: Benzimidazoles presenting intramolecular hydrogen bonding interactions have been normally used to better understand the role of H-bonding in biological processes. Here, we present an experimental and theoretical study of a new compound [2,4-di-tert-butyl-6-(3H-imidazo[4,5-c]pyridine-2-yl)phenol]; (B2), a benzimidazole derivate, exhibiting an intramolecular hydrogen bond. B2 was synthesized and characterized by its H-1, HHCOSY, FT-IR and mass spectra (EI-MS 323 M+). The electronic and optical properties of B2 were studied with theoretical calculations using density functional theory (DFT) and time-dependent DFT (TDDFT). B2 showed luminescent emission at room temperature in different solvents, with a large Stokes shift (e.g.; lambda(ex) = 335 nm; lambda(em) = 510 nm in acetonitrile). Also, the quantum yield (phi = 0.21) and theoretical band emission are reported. We found that B2 exhibited a fluorescence emission at around 500 nm in ethanol and in acetonitrile that could be quenched by aqueous solutions of Hg(NO3)(2) in the range of micro molar concentrations. Cyclic voltammetry in acetonitrile showed a strong anodic response due to a quasireversible process, with reduction and oxidation waves at -1.28 and -0.47 V vs. SCE. Regarding the biological properties, we assessed the antimicrobial activity of B2 in Salmonella enterica (bacteria), Cryptococcus spp. (yeast), Candida albicans (yeast), Candida tropicalis (yeast) and Botrytis cinerea (mold). To this end, we determined the minimal inhibitory concentration (MIC) (for bacteria and yeasts), the growth inhibition halos (for yeasts), and the inhibition of mycelial growth (for the mold). We observed that B2 exerted an antifungal effect against Cryptococcus spp. and Botrytis cinerea. In addition, due to its fluorescence properties, B2 has proven to be a suitable marker to observe bacteria (Salmonella enterica and an Escherichia coli derivative), yeasts (Candida albicans), and even human cells (SKOV-3 and HEK-293) by confocal microscopy.

Abstract: Research in fluorescence microscopy presents new challenges, especially with respect to the development of new metal-based fluorophores. In this work, new fac-[Re(CO)(3)(bpy)L]PF6 (C3) and fac[ Re(CO)(3)(dmb)L]PF6 (C4) complexes, where L is an ancillary ligand, E-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-di-tert-butylphenol, both exhibiting an intramolecular hydrogen bond, have been synthesized for use as preliminary probes for fluorescence microscopy. The complexes were characterized using chemical techniques such as UV-vis, H-1-NMR, TOCSY, FT-IR, cyclic voltammetry, mass spectrometry (EI-MS 752.22 M+ for C3 and 780.26 M+ for C4) and DFT calculations including spin-orbit effects. The electron withdrawing nature of the ancillary ligand L in C3 and C4 explains their electrochemical behavior, which shows the oxidation of Re-I at 1.84 V for C3 and at 1.88 V for C4. The UV-vis absorption and emission properties have been studied at room temperature in acetonitrile solution. The complexes show luminescent emission with a large Stokes shift (lambda(ex) = 366 nm, lambda(em) = 610 nm for C3 and lambda(ex) = 361 nm, lambda(em) = 560 nm for C4). The TDDFT calculations suggest that an experimental mixed absorption band at 360 nm could be assigned to MLCT (d(Re) -> pi*(dmb)) and LLCT (pi(L) -> pi*(dmb)) transitions. We have also assessed the cytotoxicity of C3 and C4 in an epithelial cell line (T84). We found that 12.5 μg ml(-1) of C3 or C4 is the minimum concentration needed to kill 80% of the cell population, as determined by neutral red uptake. Finally, the potential of C3 and C4 as biological dyes for use in fluorescent microscopy was assessed in bacteria (Salmonella enterica) and yeasts (Candida albicans and Cryptococcus spp.), and in an ovarian cancer cell line (SKOV-3). We found that in all cases, both C3 and C4 are suitable compounds to be used as fluorescent dyes for biological purposes. In addition, we present evidence suggesting that these rhenium(I) tricarbonyl complexes may be also useful as differential fluorescent dyes in yeasts (Candida albicans and Cryptococcus spp.), without the need for antibodies.